In a gas turbine engine, a significant safety concern is the fracture of fan blades during engine operation, that due to the high speed of rotation and relatively large mass represent a significant safety hazard as fragments are expelled at high velocity under centrifugal force. The fan casing surrounding the array of fan blades includes containment layers of energy absorbing material to absorb the impact and contain the liberated blade fragments. So called “Fan blade off events” can be caused by foreign object damage such as ingestion of birds or may be due to failure of the fan blade material.
During fracture of a fan blade, usually in the attachment fillet area joining an integral blade platform, the released fracture blade moves tangentially outwardly and collides with the following blade which continues rotation. The integral blade platform slides along the concave surface of the following blade and the platform is fractured as it progresses towards the containment layers of the fan casing. The trajectory of the fractured blade and the integral platform in particular poses the threat of substantial impact damage to the leading edge and the trailing edge of the following blade. The present invention relates to reducing the impact damage and applied loads which threaten the integrity of the trailing edge of the following blade.
As the fractured blade platform disintegrates on impact, the path of fracture of the platform often creates a curved edge that protrudes outwardly towards the trailing edge of the following blade. This curved platform fracture path creates a condition which increases the concentration of the impact force applied to the trailing edge of the following blade, thereby increasing strain in the material and increasing the likelihood of material failure originating from fractures in the relatively thin trailing edge of the following blade.
The prior art has proposed various solutions in an attempt to minimize the damage to the following blade resulting from impact with a fracture blade.
For example, U.S. Pat. No. 5,443,365 shifts the blade platform toward the following blade side of the airfoil in an attempt to permit the released blade platform to make sliding contact with the following blade that deforms and straightens the following blade to distribute impact loads more uniformly.
U.S. Pat. No. 5,836,744 to Zipps et al. provides a frangible blade platform having a groove in the platform to initiate platform fracture and a notched corner of the platform to reduce the impact strain.
On the other hand, U.S. Pat. No. 6,338,611 to Anderson et al. provides a blade platform that has been contoured to more closely match the contour of the airfoil section and thereby reduce the localization of strain induced by impact.
Disadvantages of the above prior art systems include increasing the weight of the blade significantly, increasing manufacturing costs and reducing the reliability of seals between platforms, and increasing difficulty of assembly due to increased manufacturing complexity.
It is an object of the present invention to provide the means to reduce the strain in a following fan blade during impact while having minimal effect on the overall weight of the blade and minimal increase in manufacturing cost and complexity.
Further objects of the invention will be apparent from review of the disclosure, drawings and description of the invention below.